The vast majority of prior art in the use of coupling agents in rubber involves silane molecules containing one or two, and in less frequent cases, up to several silicon atoms bound to any one of a wide number of sulfur-functional groups. Each silicon is almost exclusively bound to one or more simple hydrolyzable alkoxy group in the practiced art. Sulfur is indirectly bonded to the silicon chemically via a backbone of one to several carbon atoms. These sulfur silane coupling agents function by chemically bonding silica to polymer used in rubber applications in a relatively simple and straight forward manner. Coupling is accomplished by chemical bond formation between the silane sulfur and the polymer and by hydrolysis of the silane alkoxy groups and subsequent condensation with silica hydroxyl groups.
Commonly used coupling agents typically contain silicon exclusively bound to carbon and alkoxy groups, and need to be manufactured using anhydrous alcoholic solutions of sulfur anions of alkali metal ions or the ammonium ion. Anhydrous materials must be used so as to preserve the hydrolytically labile alkoxy groups present on silicon. Despite the low cost and general availability of hydrated sodium sulfide, polysulfide, and hydrosulfide salts, anhydrous analogs are not easily obtained or handled because of their great affinity for water. The removal of water from hydrous materials requires conditions conducive to fire hazards and usually does not go to completion. Preparations of anhydrous materials by indirect methods are costly and involve raw materials, such as metallic sodium and hydrogen sulfide, which pose special hazards during use and transportation. Thus, it would be beneficial from a safety and economic standpoint to use starting materials which do not require anhydrous conditions or reagents.
The use of mixtures of separate silicones and sulfur silanes in tire applications is known in the art and for instance, is described in European Patent EP 0 784 072 A1 wherein individual silicon containing chemical compounds are blended, either prior to incorporation into the rubber compound, or by being sequentially added to the rubber compound. Thus, it would be advantageous to have a single compound which provides the benefits obtainable from the separate silicones and sulfur silanes thereby eliminating additions during the rubber compounding.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a sulfur silane which can be made under non-anhydrous conditions.
It is another object of the present invention to provide a sulfur silane having the advantages of a both a sulfur containing silane and a silicone.